Bacteria and archaea—collectively known as prokaryotes—live pretty much everywhere, dividing happily in places from stomach acid to deep-sea vents. They can thrive in so many different places because their genomes are incredibly flexible: they can alter, lose and duplicate genes almost at will. Scientists have long recognized that prokaryotes can also acquire genes from their neighbors (a move that contributes to antibiotic resistance). But this method of gaining new DNA, termed horizontal gene transfer, was thought to be relatively rare and to occur only under strong pressures in the environment, such as exposure to powerful antibiotics.

A recent study in PLoS Genetics has found, instead, that pro­kar­y­otes acquire genes from near-by microbes quite often. This transfer, which can take place when one bug obtains genetic information from another via a bridge or a virus, can happen even when the two prokaryotes are from different species.

By compiling a database of 110 different prokaryote genomes, Todd J. Treangen and Eduardo P. C. Rocha of the Pasteur Institute in Paris calculated the number of genes that had been acquired through horizontal gene transfer. They knew that genes that evolve within a pro­karyote’s own genome are often located near similar genes and have similar functions in existing genes. Genes that arrive via horizontal transfer, however, appear randomly throughout the genome and often have radically different functions. By tracking these two major markers, Treangen and Rocha calculated that the prokaryotes they studied acquired between 88 and 98 percent of new genes through horizontal transfer.

“This [study] shows that most new genes in bacteria are gained from outside sources,” says Howard Ochman, a microbiologist at Yale University, who was not involved in the research. “It took full genome sequences and a lot of good sense about how to cull the data, and I think that their conclusions are really robust.” 

After arriving in their new homes, transferred genes follow a different evolutionary path than do genes that evolve within the same genome. The newcomers evolve more quickly and stay longer—behaviors that Treangen suspects arise because the genes provide radical new and useful functionality.

Horizontal gene transfer allows pro­kary­otes to acquire “preexisting adaptations from other microbes,” says Treangen, which enables them to rapidly establish themselves in new environments. This study, he asserts, shows that horizontal transfer is the dominant force driving prokaryote evolution and helps to explain why bacteria have developed antibiotic resistance so quickly.